Thanks all for your replys.
Firstly, I may not have made it clear. I don't want to be with anyone else and she doesn't want me to be with anyone else either. But she would try to understand and let me (with difficulty) if that's what I wanted. So she's not too selfish in that respect. She knows it's tough for me but she's trying to be honest with how she's feeling.
She says, after much talking, that maybe the feeling will pass. But as I love her so much I don't want her feeling this way if its what she needs. We've decided to wait a while and she how she feels. I admit I am hurt that its something I can't satisfy but I do actually understand where shes coming from. I know it'll only be a physical thing but I don't know how I'll feel about her afterwards. I hope ok. Also I'm worried that this might open some floodgates with her and she might not want to look back. She says she doesn't think so but did ask me what if she did feel like this. I think I'd want out of the relationship, in fact I know I would. This just isn't for me. Especially the way it is, ie: she'd never want me to be there, she's not in to 3somes or more.
Should I try to understand and let her get it out of her system if she still feels this way or make a stand. She says she won't do it if I don't want her to but I don't want her to stay feeling like this. I'd prefer she did what she had to do and get it out of her system, even if it means us splitting up!
I really don't know!